### Added
- **PlaceholderAPI Integration:** Full support for PlaceholderAPI with 30+ placeholders for scoreboards, holograms, and other plugins.
- Market data: `%endex_price_<MATERIAL>%`, `%endex_trend_<MATERIAL>%`, `%endex_supply_<MATERIAL>%`, `%endex_demand_<MATERIAL>%`
- Top items: `%endex_top_price_<1-10>%`, `%endex_bottom_price_<1-10>%`, `%endex_top_gainer_<1-10>%`, `%endex_top_loser_<1-10>%`
- Holdings: `%endex_holdings_total%`, `%endex_holdings_count%`, `%endex_top_holdings_<1-10>%`
- Statistics: `%endex_total_items%`, `%endex_total_volume%`, `%endex_average_price%`, `%endex_active_events%`

- **Update Checker:** Automatic update notifications for server administrators.
- Checks Spigot and Modrinth for new versions on startup.
- Console banner when updates are available.
- OP players notified on join when updates available.
- Configurable via `update-checker.enabled` and `update-checker.notify-ops`.

- **GUI Customization System:** Full GUI layout and appearance customization.
- Per-GUI config files in `guis/` folder: `market.yml`, `details.yml`, `holdings.yml`, `deliveries.yml`.
- Customizable titles, sizes, and slot positions.
- Category definitions with custom icons and materials.
- Background filler and navigation button customization.

- **Command Aliases System:** Custom command alias registration.
- Configure aliases in `commands.yml` for any market subcommand.
- Example: `/shop` → `/market`, `/stock` → `/market holdings`.
- Supports tab completion inheritance.

### Configuration
New config files added:
```yaml
# config.yml additions
update-checker:
enabled: true
notify-ops: true

# guis/market.yml, details.yml, holdings.yml, deliveries.yml
# commands.yml for aliases
```

### Dependencies
- PlaceholderAPI added as soft dependency (optional but recommended).

### Technical
- `EndexExpansion` class for PlaceholderAPI expansion.
- `UpdateChecker` class with Spigot API and Modrinth API integration.
- `GuiConfigManager` for loading and managing GUI configurations.
- `CommandAliasManager` for dynamic command alias registration.
- `SqliteStore.getAllPlayersHoldings()` method for leaderboard placeholders.